Project Summary Investigating mechanisms of specificity in a vertebrate-bacteria symbiosis Despite an increased awareness of the importance of the microbiome to human health and disease, relatively little is known about the molecular mechanisms employed by symbiotic bacteria to stably colonize the gut, as they are difficult to disentangle and experimentally study in isolation. There are several established model systems used to explore specific pathways and processes underpinning symbiotic associations, however, none represent a naturally evolved, binary symbiosis between a vertebrate host and a single bacterial species. A major, long-term objective of this study is to establish the bioluminescent symbiosis between a coral reef fish (Siphamia tubifer) and a luminous bacterium in the Vibrio family, (Photobacterium mandapamensis) as a model association to define the mechanisms involved in regulating specificity and colonization in a gut-associated symbiosis. The results of this project will provide new insights on the evolution of stable interactions between vertebrate hosts and beneficial bacteria and can be compared across model hosts to further define the universal principals underlying animal-microbe associations. The overall objective of this project is to investigate how the specificity of vertebrate-bacteria associations is maintained from a broad evolutionary scale down to the molecular level by addressing the following research aims: 1) Define the degree of specificity of the Siphamia?Photobacterium symbiosis, 2) Characterize the infection dynamics and symbiont competition within a host light organ, and 3) Identify key mechanisms involved in the establishment and persistence of the symbiosis. To do so, the specificity of the symbiosis will first be broadly defined across all 23 species in the host fish genus, then across the broad geographic distribution of a single host species, S tubifer. The specificity of the S. tubifer-P. mandapamensis association will then be tested in culture to determine whether it is regulated by local environmental and ecological factors or conserved at a more molecular level. Next, the intra-species symbiont diversity will be experimentally tested to better understand strain competition and infection dynamics within a host. Finally, the genetic mechanisms involved in regulating the symbiosis will be determined by comparing both host and symbiont gene expression throughout the infection process. Overall this study will reveal the processes that regulate the establishment and maintenance of specific associations between vertebrate hosts and beneficial bacteria across multiple timescales, and in doing so, will provide the greater research community with a novel binary vertebrate-bacteria model system with which to deepen our understanding of these vital interactions.